Some signaling schemes, such as Gunning Transistor Logic (GTL) and low-swing current-mode signaling schemes, require termination resistors for the signal transmission lines. Traditionally, termination resistors are placed on the Printed Circuit Board (PCB).
FIG. 1 shows a conventional termination circuit. The termination circuit includes resistors (11 and 13) that are connected in serial between two power supply voltages: Vcc (17) and Ground (15). When each of the two resistors (11 and 13) has impedance of 2×R, the termination circuit provides termination impedance of R for the receiver input, which is to be connected between the two resistors (11 and 13). In FIG. 1, the voltage of the receiver input point is biased by the resistors (11 and 13) at the operation point of Vcc/2.
Recently, termination resistors have been placed on the silicon chip. Termination resistors integrated on the semiconductive substrate can be implemented via a poly resistor, a PMOS transistor, a combination of poly resistor and a PMOS transistor, a parallel NMOS and PMOS structure, etc. For example, Yongping Fan and Jeffrey E. Smith presented a resistor circuit for an on-die termination circuit in “On-Die Termination Resistors with Analog Impedance Control for Standard CMOS Technology, IEEE Journal of Solid-State Circuits, Vol. 38, No. 2, February 2003, pp. 361-364.
When the termination circuit is implemented via passive resistors, the linearity of the termination circuit is excellent. The current consumed by the termination circuit is almost constant, which can be expressed by the equation: It=Vcc/(4×R). When the termination resistors are implemented using other techniques (e.g., the resistor circuit of Yongping Fan and Jeffrey E. Smith), the termination circuit consumes a similar amount of power.
A highly integrated circuit chip may include a large number of input/output (I/O) pins that need termination circuits. The termination circuits of the I/O pins can consume a significant amount of power.